1. Field of the Invention
The present invention relates to an illumination apparatus for externally applying a high frequency wave electromagnetic field to a discharge tube, thereby causing light emission of the discharge tube.
More particularly, the present invention relates to an illumination apparatus suitable as a light source for original exposure in an original reading system.
2. Related Background Art
Fluorescent and halogen lamps have been very popular as conventional illumination apparatuses for original exposure in original reading systems or the like.
The fluorescent lamp emits a small quantity of light and is used as a conventional illumination apparatus for a low-speed original reading system. If the fluorescent lamp is used as an illumination apparatus for a high-speed original reading system by increasing power supply and brightness (i.e., the quantity of emission light), a filament arranged inside the fluorescent tube is melted. Therefore, the level of power supplied to the fluorescent lamp is limited, and the fluorescent lamp is not suitable for a high-speed original reading system.
The halogen lamp has a large quantity of emission light and has been used in an original reading system. However, the halogen lamp generates infrared rays having wavelengths outside the wavelengths of visible light range required for reading the original. Power consumption of the halogen lamp is large, and luminous efficiency thereof is low. In addition, the infrared rays generate heat, which requires a cooling unit, and in particular, a large cooling unit. Therefore, the halogen lamp is not suitable as a compact, inexpensive, low-power consumption illumination apparatus.
The present inventors have already proposed illumination apparatuses for generating a large quantity of emission light in the visible light range so as to solve the above problem, as disclosed in copending U.S. patent application Ser. Nos. 944,863 and 942,833.
As shown in FIGS. 6 and 7, each illumination apparatus of U.S. Ser. Nos. 944,863 and 942,833, comprises a discharge tube 1 for emitting light in response to a high frequency electromagnetic field, electrodes 2 mounted on the outer wall surface of the discharge tube 1, and a high frequency wave applying means 3 for applying a high frequency wave to the electrodes 2.
In a conventional illumination apparatus for causing the discharge lamp to emit light upon an external application of a high frequency wave electromagnetic field to the charge tube, high power can be applied to the electrodes. In addition, such an illumination apparatus is suitable as an exposure light source in an original reading system so as to primarily generate light in the visible range.
However, in this illumination apparatus, ultraviolet rays caused by oxidation of a discharge initiator such as mercury inside the discharge tube serves as a light emission source. If a temperature in the discharge tube varies, the vapor pressure of the discharge initiator gas and hence luminous efficiency of ultraviolet rays is changed. As a result, luminous efficiency varies.
The present inventors found that the quantity of emission light varied if variations in luminous efficiency caused changes in temperature of the discharge tube.